Some vehicles of today are arranged to use a fuel gas or a fuel gas mix for generating an operational torque by means of a combustion engine. A fuel gas tank of such a vehicle may be refuelled manually at gas stations.
Examples of such a fuel gas can be natural gas, biogas, and town gas. Since a composition of such a fuel gas may vary depending on at which site said vehicle is refuelled e.g. proper engine operation may be difficult to optimize. In order to effectively, environmental friendly and safely operate said engine it is desired to determine a prevailing composition of said fuel gas. Examples of gas compositions of this fuel gas are methane, ethane, propane, butane, pentane, N2, and CO2. It is today difficult to accurately determine a prevailing composition of a gas mix of a fuel tank of a vehicle at a reasonable price.